The present invention is concerned with providing the driver of a vehicle fitted with an electro-hydraulic braking system with a warning of the existence of a braking malfunction condition.
A typical EHB system for a vehicle comprises a brake pedal, respective braking devices connected to the vehicle wheels and which are capable of being brought into communication with electronically controlled proportional control valves in order to apply hydraulic fluid under pressure to the braking devices, a hydraulic pump driven by an electric motor, and a high pressure hydraulic pressure accumulator fed by said pump for the provision of hydraulic fluid under pressure which can be passed to the braking devices via the proportional control valves in order to apply hydraulic fluid under pressure to the braking devices in so called xe2x80x9cbrake by wirexe2x80x9d mode in proportion to the driver""s braking demand as sensed at the brake pedal. The EHB system is controlled by an electronic controller (ECU).
In order to make the EHB system xe2x80x9cfeelxe2x80x9d like a conventional braking system, a travel simulator is also provided which is connected hydraulically to a master cylinder coupled to the brake pedal and which allows, by increasing the volume under pressure, the brake pedal to be depressed to an extent comparable with that of conventional systems. In the event of failure of the brake-by-wire mode, the system can adopt a so-called xe2x80x9cpush-throughxe2x80x9d mode in which at least the front brakes are controlled directly from the latter master cylinder. In this mode of braking, it is usual to isolate the travel simulator by the closure of an electronically operated valve, in order to reduce the pedal travel required to apply the desired braking pressure.
A problem with known EHB systems is that they can continue to operate with the existence of a (non-critical) fault condition but without the driver feeling any difference in the pedal behaviour and therefore being unable to detect the fault condition by any change in the xe2x80x9cfeelxe2x80x9d of the system. In such cases, the normal approach is to illuminate a dashboard warning lamp. However, this may not be noticed by the driver or, it may be noticed, but ignored because its significance is not appreciated.
In accordance with the present invention, it is arranged that, if such a (non-critical) fault condition arises, the driver is caused to experience a different to usual travel/force behaviour characteristic through the braking pedal when braking. Thus, for example, the driver can experience a different-to-usual braking effect for a given pedal depression. This provides optimum feedback as the driver will have his attention focused on such a different (unusual) pedal behaviour.
In one embodiment, the modified travel/force behaviour characteristic can be arranged to give the driver a lower deceleration of the vehicle than usual for a given pedal travel, so that the driver has to press the brake pedal harder than usual.
In another embodiment, the modified travel/force behaviour characteristic can be arranged to give the driver a greater deceleration of the vehicle than usual for a given pedal travel, so that the driver has to press the brake pedal less hard than usual.
In the latter case, the driver can also be arranged to experience a strong brakes xe2x80x9cjump-inxe2x80x9d in the initial range of pedal travel.
In a still further embodiment, the modified travel/force behaviour characteristic can be arranged deliberately to induce oscillations in the brake pressure so as to create audible noise and/or vibrations in the vehicle chassis to produce driver discomfort. The level of discomfort can be arranged to be increased if the fault continues to be ignored over time.
Preferably, the modified travel/force behaviour characteristic is constrained to lie within a boundary limited by the maximum pedal travel (Smax) and maximum braking pressure (Pmax) allowable by the system under normal braking conditions.
The responsibility for changing the pedal behaviour in dependence on a fault condition can be borne by a control logic which can be implemented in the electronic control unit(s) of the system. This has the advantage that an intervention in the system hardware is not required. Generally, the modified characteristic would not be obtained by alteration of the pedal simulator circuit, although it could in principle, but is achieved rather by modified control of actuating valves/solenoids by the CPU within the system which determines the overall braking effect for a given pedal displacement, including for example the proportional solenoid control valves 14.
The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings.